The present invention relates to a rewritable optical recording medium, particularly, a magneto-optical recording medium in which the direction of magnetization can be changed by the irradiation of a laser beam in the magnetic field, resulting in recording, reproducing and erasing of information. More particularly, the present invention relates to a surface reading type magneto-optical recording medium which is recorded and reproduced by the irradiation of a laser beam on the surface of the medium.
A magneto-optical recording medium is a portable recording medium having a large capacity and high-density. In these years, demand for a magneto-optical recording medium as a rewritable medium has arisen on the purpose for recording files with a large capacity used for computers or data of moving pictures, with the development of the recent multi-media system.
A magneto-optical recording medium is generally composed of a multi-layer including a recording layer formed on a substrate (a transparent circular disk) made of such as plastics. A magneto-optical recording medium is recorded and erased by an irradiation of a laser beam in a magnetic field and reproduced by a reflection light of a laser. As for recording method, formerly, the so-called light intensity modulation recording method is widely conducted, in which conducted erasing a record in a fixed magnetic field followed by recording in a magnetic field fixed in an opposite direction. Lately, a magnetic field modulation recording method is attracting public attention, in which method a magnetic field is modulated in accordance with a recording pattern while irradiation of a laser beam. Because, the above magnetic field modulation recording method is a method capable to record (direct overwriting) with high accuracy and high density in one revolution.
Further, in recent years, a recording and reproducing laser beam was irradiated on a recording layer through a substrate. Currently, as a means of a recording with high density, the so-called near-field optical recording method is attracting public attention, in which method recording and reproducing are operated by an optical head placed adjacent to a recording film (Appl. Phys. Lett. 68,p.141 (1996)). In the above recording method, Solid Immersion Lens (hereinafter referred to as SIL) head is used in order to minimize a focused laser beam spot size. Therefore it is possible to reproduce a mark shorter than that of the former recording limit (xcx9cxcex/2NA, wherein NA indicates the numerical aperture of the objective lens) determined by the wavelength (xcex) of a laser beam of a light source, resulting in the achievement of recording and reproducing with super high recording density. In near-field optical recording, it is necessary that an optical head is placed adjacent (20 to 200 nm) to an optical medium. Therefore, a laser beam is irradiated on a recording layer directly not through a substrate, not like as a current magneto-optical recording medium with which a laser beam is irradiated on a recording layer through a substrate. That is, a current magneto-optical recording medium generally has a structure of substrate/primary dielectric layer/recording layer/secondary dielectric layer/reflection layer in this order. On the other hand, a magneto-optical recording medium used for near-field optical recording has a structure of substrate/reflection layer/primary dielectric layer/recording layer/secondary dielectric layer in an opposite order to that of the current one. Using a near-field magneto-optical recording medium, a laser beam is irradiated on the top surface of a multi-layer for recording and reproducing (surface reading type recording). In this case, a flying type slider head is often used in order to close a recording layer to a SIL head. As for recording method, it is said that a magnetic field modulation recording is suitable for a near-field magneto-optical recording. In the above magnetic field modulation recording, a recording layer is heated at a temperature higher than Curie temperature of the material by the irradiation of a laser beam and a magnetic field is modulated by a thin film coil formed on a slider head.
In a near-field magneto-optical recording, observed are unfavorable phenomena such as a disordered tracking error signal and adhesion of dust particles while irradiating a recording laser beam, because of the narrow space between a SIL head and the surface of a medium.
The object of the present invention is to provide a near-field magneto-optical recording medium having excellent SNR, resolution, and high recording sensitivity, and which excellent in recording with a short track pitch. Particularly, the object of the present invention is to provide a near-field magneto-optical recording medium capable of stable and reliable recording by the irradiation of a laser beam.
Under these circumstances, the present inventors regarded as follows. The heat in a recording layer generated by a laser power in recording is diffused through a dielectric layer up to the surface of a medium. Then adsorbates and minute dust particles adhere to the surface of a medium are heated and scattered causing the occurrence of disordered optical signals. Also, in the area just under SIL lens, the density of power is extraordinary high. Therefore, if dust particles and defects placed on the surface of a medium or on the optical path under SIL lens, absorbed a laser beam, they were heated unusually causing disordered optical signals.
From above, inventors have made extensive and intensive studies about the structure of a medium, and as a result, found that the object can be solved by inhibiting an increase in temperature of the surface of a medium when a recording layer is heated by irradiation of a laser beam and decreasing the temperature of the surface immediately after irradiation.
That is, the above object can be solved by insulating heat of a recording layer generated by a laser power in recording by use of a thick dielectric layer from the surface of a medium. Namely, the above thermal insulation can be achieved by forming a thick dielectric layer having appropriate characteristics of insulation and thermal diffusion. Also, the object can be solved by forming a two-layered dielectric layer. In the above two-layered dielectric layer, lower dielectric layer next to a recording layer shows thermal insulation effect inhibiting thermal diffusion from a recording layer to the surface of a medium, and upper dielectric layer thereon shows thermal diffusion effect which promote cooling the surface of a medium, adsorbates and minute dust particles adhered to the surface of a medium. From the above, the present invention has been completed.
The first magneto-optical recording medium of the present invention is a magneto-optical recording medium characterized in a plurality of layers comprising at least a reflection layer, a recording layer and a dielectric layer laminated on a substrate in the above order, wherein the thickness of the dielectric layer is not less than 140 nm and not more than 1,000 nm.
The second magneto-optical recording medium of the present invention is a magneto-optical recording medium characterized in a plurality of layers comprising at least a reflection layer, a recording layer, a lower dielectric layer, an upper dielectric layer, and a solid lubricating layer laminated on a substrate in the above order, wherein the thermal conductivity of the upper dielectric layer is higher than that of the lower dielectric layer, and the total thickness of the lower dielectric layer, the upper dielectric layer and the solid lubricating layer is not less than 90 nm and not more than 2,000 nm.
A magneto-optical recording medium of the present invention can be applied in a surface reading type magneto-optical recording medium used in a near-field magneto-optical recording in which it is necessary that an optical head is placed adjacent to a recording medium. The above surface reading type magneto-optical recording medium is used for recording and reproducing by irradiation of a light from the surface of a medium.